Effects of Turbulence on Aerofoils at High Incidence

Abstract

Two effects of free stream turbulence on aerofoils at incidence were investigated: (1) The mean loading effect. A survey of the literature dealing with the effects of turbulence on aerofoil mean loading and stalling was undertaken. This survey was supplemented by the results of an experiment in which three symmetric aerofoils, of different thicknesses, were tested in various turbulent flows. The effects of turbulence intensity and turbulence length scale were examined separately. For aerofoils with ultimate leading edge stall, it was confirmed that the effect of turbulence on the maximum lift is approximately equivalent to the effect of increased Reynolds Number. The variation of this maximum lift with length scale was found to be greater than the -1/5 power law, suggested by a theory of G. I. Taylor. (2) The unsteady loading effect. The higher order unsteady loads arising from the incidence of the aerofoil were investigated from both the theoretical and experimental viewpoints. The higher order theory, which includes the effect of gust distortion by the mean flow field, was initially developed for a sinusoidal gust. This result was employed in two ways: (a) It was applied directly to the problem of the periodic loading of turbomachine blades. (b.) it was used in the formulation, via Fourier analysis, of a higher order loading theory for an isotropic turbulent gust. This latter theory predicts that the higher order admittance of lift (lift spectrum ÷ upwash spectrum) depends on the incidence, infinity , to the second power. In the experiment, unsteady loads were measured on a rigid model at incidence ÷, and subjected to an approximately isotropic turbulent flow. The measured admittance of lift, due to incidence, was in reasonable agreement with the theoretical prediction.